docs(runtime-config): clarify how environment variables are loaded

docs/content/2.guide/2.features/10.runtime-config.md

@@ -1,6 +1,6 @@
 # Runtime Config
 
-Nuxt provides a runtime config API to expose config within your application and server routes with the ability to update them at runtime using environment variables.
+Nuxt provides a runtime config API to expose configuration within your application and server routes, with the ability to update it at runtime by setting environment variables.
 
 ## Exposing runtime config
 
@@ -35,11 +35,15 @@ console.log(runtimeConfig.public.apiBase)
 The most common way to provide configuration is by using [Environment Variables](https://medium.com/chingu/an-introduction-to-environment-variables-and-how-to-use-them-f602f66d15fa).
 
 ::alert{type=info}
-Nuxt CLI has built-in [dotenv](https://github.com/motdotla/dotenv) support.
+Nuxt CLI has built-in [dotenv](https://github.com/motdotla/dotenv) support in development mode and when running `nuxi build` and `nuxi generate`.
 
-In addition to any process environment variables, if you have a `.env` file in your project root directory, it will be automatically loaded into `process.env` and accessible within your `nuxt.config` file and modules.
+In addition to any process environment variables, if you have a `.env` file in your project root directory, it will be automatically loaded **at build/dev/generate time**, and any environment variables set there will be accessible within your `nuxt.config` file and modules.
 
-However, **after your project is built**, you are responsible for setting environment variables when you run the server - your `.env` file will not be read at this point. For example, you could pass the environment variables as arguments using the terminal `DATABASE_HOST=mydatabaseconnectionstring node .output/server/index.mjs`.
+When updating `.env` in development mode, the Nuxt instance is automatically restarted to apply new values to the `process.env`.
+
+However, **after your server is built**, you are responsible for setting environment variables when you run the server. Your `.env` file will not be read at this point. How you do this is different for every environment. On a Linux server, you could pass the environment variables as arguments using the terminal `DATABASE_HOST=mydatabaseconnectionstring node .output/server/index.mjs`. Or you could source your env file using `source .env && node .output/server/index.mjs`.
+
+Note that for a purely static site, it is not possible to set runtime configuration config after your project is prerendered.
 ::
 
 Runtime config values are automatically replaced by matching environment variables at runtime. For this to work, you _must_ have a fallback value (which can just be an empty string) defined in your `nuxt.config`.